Phenalkamines are epoxy hardening agents comprised of cardanol, formaldehyde, and an organic diamine. Cardanol is produced from Cashew Nutshell Liquid (CNSL). Phenalkamine may be produced via a Mannich reaction to produce a low molecular weight polymer by condensing one mol of alkyl phenol, two mols of formaldehyde and two mols of a polyamine. The polyamine may be aromatic or aliphatic. Commercially, ethylene diamine and diethylenetriamine are used to produce Cardolite NC541 and Cardolite NC 540, respectively.
Phenalkamines are products of the reaction (condensation products) of cardanol (I), which, chemically, is a C15 alkylphenol and a major constituent of the oil obtainable from CNSL, with aliphatic (primary or secondary) amines and formaldehyde.
where n=0, 2, 4, 6 (2 times the number of double bonds).
The condensation step is carried out by first adding formaldehyde to the reaction, in the presence of at least a catalytic amount of an amine, thereby producing an alkyl C15 phenol methylol pre-polymer, followed by condensation with a polyamine, which releases water. The temperature during synthesis should be less than 90° C. to minimize color development.
Further information on phenalkamines can be found in the publication: Zhishen Dat et al., “Phenalkamines: Multipurpose Epoxy Curing Agent”; Cardolite Corporation, Newark, N.J., USA; Reprint EPI-ERF Conference, September 1994.
Crude CNSL predominantly contains anacardic acid (II). The distillation of CNSL in the presence of acid gives a composition which mainly contains cardanol and, as a secondary product, cardol (III), see, for example, U.S. Pat. No. 6,262,148, and U.S. Pat. No. 6,229,054. It has been found that the distillation of crude CNSL provides a composition which mainly contains cardanol and, as a secondary product, cardol and small quantities of 2-methyl cardol and anacardic acid.

The cardanol/cardol mixture thus obtained has three technical disadvantages: (1) production by distillation from crude CNSL involves losses of part of the cardanol through polymerization so that, ultimately, the yield of cardanol in the distillate is only 50-60%; (2) the initially pale yellowish cardanol/cardol mixture changes during storage, rapidly turning brown in color, which is attributed to the presence of cardol; and (3) products ensuing from the cardanol/cardol mixture also undergo unwanted changes in color during storage.
The resulting product is effective as an epoxy hardener, but it has a dark color, based on the Gardner scale, of greater than 12. The dark color in the existing product is due to impurities, including cardols (a di-hydroxy alkyl phenol) which lead to rapid polymerization and oxidation reactions, which generate precursor color bodies, either during storage or during phenalkamine manufacturing. Current manufacturing processes do not separate the cardol impurities at a satisfactory level in which no darkening occurs. In addition, at low temperatures, i.e., 0° Celsius, the curing speed is slow.
There remains a need for epoxy resin curing agents that effectively cure at reduced temperatures.